In bleaching processes for both softwood and hardwood pulps, the pulps are normally delignified in one or more oxygen steps and thereafter bleached by means of various sequences comprising chlorine dioxide steps, extraction steps, peroxide steps, etc.
Hardwood pulps differ from softwood pulps in that they contain high amounts of Hexenuronic Acid (HexA). The amount of HexA depends on the raw material used and the cooking conditions. Modern methods of cooking, which utilize relatively low cooking temperatures, normally render high contents of HexA. HexA is oxidized by potassium permanganate (KMNO4) and thereby contributes to the kappa number. In a hardwood pulp with a kappa value of 10, 50 to 70% of the kappa value could be a result of HexA and only 30 to 50% is attributed to lignin and other compounds.
During bleaching, HexA can be reduced by oxidation with bleaching chemicals such as chlorine dioxide and ozone. A more economical way to do so is to degrade HexA by means of acid hydrolysis at high temperature, which lowers the amount of double bonds in the remaining pulp. Therefore, a hot chlorine dioxide step (DHT) is often accommodated in modern bleach plants. In this stage both oxidation and acid hydrolysis are performed. The high temperature in DHT can provide a reduction of the kappa number from for example 10.5 to 2.5. Hence, most of the reduction of the kappa number, typically 85 to 90%, is achieved in such a DHT-step and only a minor part, typically 10 to 15%, in a following extraction step (E). Moreover, it is believed that lignin is also degraded into smaller, more water soluble pieces during the DHT-step.
Swedish Patent No. 466,062 discloses a method of bleaching a chemical pulp in a sequence comprising at least four bleaching steps, with final bleaching in a first and a second chlorine dioxide step. Between the chlorine dioxide steps an alkaline extraction is carried out and washing takes place between the first chlorine dioxide step and extraction. Immediately after said washing step, NaOH is charged in an amount of 4 to 10 kg/ton pulp. Thereafter, an oxidizing agent is admixed in an amount of up to 2 kg/ton pulp. An acid is added for lowering the pH-value, but without effecting a complete neutralization of residual alkaline.
Swedish Patent No. 526,162 discloses a bleaching process for hardwood pulp wherein an oxygen-delignified and washed pulp is subjected to a chlorine dioxide bleaching step at high temperature, such as at least 90° C., and treated with a chelating agent without any intermediate wash. The pulp is thereafter washed and subjected to a pressurized peroxide bleaching step in which alkali is also added. The bleached pulp is finally washed in order to obtain a pulp with a brightness of 88 to 90% ISO.
International Application No. WO 02/075046 discloses a method for end bleaching of pulp comprising two chlorine dioxide steps. The pulp is washed and dewatered after the first chlorine dioxide step to a concentration of 12 to 50% in order to remove dissolved metal ions. Thereafter, alkali is added for extraction and rapid increase of the pH. Before the pulp is introduced into the second chlorine dioxide step, acid and chlorine are added to the pulp.
The previously known methods generally perform well, even though they may be fairly expensive or complex. Nonetheless, there remains a desire to further improve the bleaching, especially for hardwood pulps, and reduce the overall costs for the bleaching.
Hence, one object of the present invention is to provide a method for bleaching a hardwood pulp to a brightness of from about 88 to 92% ISO in a cost effective manner.